MOTIVE POWER OF HEAT. 61 



other hand, the substance acted upon by the heat. 

 The motive power of a waterfall depends on its 

 height and on the quantity of the liquid; the 

 motive power of heat depends also on the quantity 

 of caloric used, and on what may be termed, on 

 what in fact we will call, the height of its fall,* 

 that is to say, the difference of temperature of the 

 bodies between which the exchange of caloric is 

 made. In the waterfall the motive power is ex- 

 actly proportional to the difference of level between 

 the higher and lower reservoirs. In the fall of 

 caloric the motive power undoubtedly increases 

 with the difference of temperature between the 

 warm and the cold bodies ; but we do not know 

 whether it is proportional to this difference. We 

 do not know, for example, whether the fall of ca- 

 loric from 100 to 50 degrees furnishes more or less 

 motive power than the fall of this same caloric from 

 50 to zero. It is a question which we propose to 

 examine hereafter. 



We shall give here a second demonstration of 

 the fundamental proposition enunciated on page 

 56, and present this proposition under a more gen- 

 eral form than the one already given. 



* The matter here dealt with being entirely new, we are 

 obliged to employ expressions not in use as yet, and which 

 perhaps are less clear than is desirable. 



